Even in the simplest form in which we know it, the legend has been much transformed by poetic fancy. As a heroine united with the country from immemorial time, Io is called daughter of Inachus, the river of Argos and its oldest king, or of Iasus, from whom comes the epithet *Iασov *Apγos. As associated with the oldest worship of Hera, she is called the daughter of Peiras, who made the first image of the goddesg out of a pear tree at Tiryns; and Io Callithyia is, by a common device in such legends, the first priestess of the goddess. Zeus fell in love with her, and she was transformed into a white cow either by Zeus, to hide her from the rage of Hera, or by the jealous goddess herself. When Io and Hera had once been made into distinct personalities, such tales easily arose to explain the relation between them. Hera then insisted on getting this cow from Zeus, and set Argus Panoptes with his thousand eyes to watch her. Io is almost universally understood to be the moon, and Argus the star-studded nightly heaven. Argus tied the cow to the olive tree shown in the sacred grove on Mount Euboea, or according to the poets pastured her in the fertile meadows of Lerna or Nemea. Zeus now sends his messenger Hermes, who lulls Argos to sleep with his magic wand, and slays him with the same curved sword, harpe, with which afterwards Perseus, the light-hero, slew the Gorgon, the power of darkness. According to another account Argus, the darkness, is slain by a stone thrown by Hermes, i.e., by the rising sun, whose sudden appearance is frequently spoken of as the throwing of a stone (Kuhn, Entwickl. d. Mytholj}. Maddened by a gadfly, Io wanders over many lands till at last she comes to Egypt, where she regains human form and becomes the mother of Epaphus. Opinions differ much as to the interpretation of this part of the tale. It is not probable that both Zeus and Hermes figured in the original legend; and the end has certainly been adapted so as to bring Greece and Egypt into connexion, and dates therefore from the time when intercourse between them became frequent and much influence was exerted by Egyptian religion on Greek thought, 7.e., the 7th century b.c. How far Oriental influence had affected the cultus at the period to which the origin of the legend belongs is doubtful ∙ i Preller compares the Phoenician conception of Astarte as a wandering cow. In later time Isis, who was conceived as horned (Herod, ii. 41), was connected with Io. The legend of Io was a favourite subject among Greek painters, and many representations are preserved on vases and in wall paintings (see Overbeck, Kunstmyth. d. Zeus, 465).

See the works quoted under Hermes ; and against the explanation of Io as the moon see Plew in N. Jαhrbb., 1870 and 1873.

Recent investigations have disclosed a number of most remarkable facts regarding the behaviour of iodine, and the allied elements bromine and chlorine, which merit a brief description here. Free chlorine, bromine, and iodine are respectively represented by the formulae Cl 2 , Br 2 , and I 2 ; that is to say, their molecules are “diatomic ,” each consisting of two atoms (comp. voL v. pp. 467-472). On the other hand, the molecules of which sulphur vapour at a temperature of about 500° C. consists are hexatomic, as expressed by the formula S 6 ; but on raising the temperature these molecules undergo simplification, so that at temperatures above 800° the vapour appears to consist entirely of diatomic molecules such as are indicated by the formula S 2 . It would seem that the halogens undergo a similar molecular simplification when heated.

Having devised a method of extreme simplicity for the determination of vapour density, V. Meyer was led in the summer of 1879 to determine the density of a number of elementary bodies at much higher temperatures than had previously been employed, and among others chlorine was examined. He was then led (in conjunction with C. Meyer) to the discovery that at high temperatures this gas has a very much lower density than corresponds to the formula Cl 2 {Berichte der deutschen chemischen Gesellschaft zu Berlin, 1879, p. 1430; comp. ibid, 1880, p. 1172). Subsequently he extended his observations to bromine and iodine (ibid., 1880, p. 394), and with similar results. Meier and Crafts took up the subject with the object of verifying V. Meyer’s statements (ibid., 1880, p. 851); they introduced several refinements in the method of operating, and determined the temperatures at which the experiments were made more accurately; in the main, however, their observations with iodine were confirmatory of V. Meyer’s. V. Meyer’s original results, and those of Meier and Crafts, are arranged in the following table, where the numbers in the column headed indicate the ratio between the observed density and the theoretical density on the air scale corresponding to the formula I 2 (8 - 79). [table]

Meier and Crafts were of opinion that the highest temperature they employed was probably as high as that estimated by V. Meyer at 1570°, and the latter chemist subsequently acknowledged the justice of their criticism of his determinations of temperature, which were conducted by a calorimetric method, whereas Meier and Crafts employed an air thermometer. V. Meyer has since extended his observations to a still higher temperature, and has obtained the values 4 , 53, 4 ’ 55, 4 ’ 57, which are not far removed from the theoretical value 4 - 39, corresponding to the formula I for the iodine molecule (op. cit., 1880, p. 1010).

An important series of observations by Meier and Crafts (Comptes Pendus, xcii. 39) on the density of iodine at various temperatures under various pressures show that at temperature below 700° and pressures below atmospheric pressure the density is constant, and corresponds to the formula I 2 , and that the density diminishes more rapidly with rise of temperature.

From the earlier results obtained by Meier and Crafts, A. Naumann has calculated the rate of dissociation of iodine, on the assumption that the decomposition is expressed by the equation I 2 = I +1, and has shown that it is in accordance with the general law of dissociation deduced from the dynamical theory of gases. He points out as especially remarkable that dissociation probably extends over 1200°, since it is only half completed at a temperature of about 1270°, and commences at least 600° lower.

The observations of Meier and Crafts indicate that the density of iodine begins to be abnormal at a temperature between 600° and 700°. The dissociation of bromine apparently does not commence at so low a temperature, and at a temperature at which the ratio of the observed to the theoretical density is , 66 for iodine, it is - 8 for bromine. Chlorine is much less readily dissociated than bromine. These results are in accordance with the general chemical behaviour of the halogens. It has yet to be proved, however, that the dissociation is of the character indicated above, and that the molecules of the halogens do not undergo a less simple decomposition such as is contemplated in Sir Benjamin Brodie’s calculus of chemical operations. (h. e. a.)